Hm i think there is no use for watercult at all Chazar
.
The 50 points are e.g. 1 scale and something .
If you play without water nations there are then much cheaper ways to kill the indy water provine guards like using an ID or some mech men .
All your watermages can go into the sea also and almost every nation can forge a waterbreath item which is enough to bring with enough good troops to kill the underwater indies .
Finally you will likely find a coast province with shamblers or other amphibian creatures which are enough to conquer the water also .
If Water cult would grant Water breathing to all your units then i would consider it . But as it is you better invest the points in an additional scale or something similiar .